The context for this illustration is rather lovely.  A Jewish mystic fixes a precious opal in a frame "not unlike that of a looking glass," hangs it on a silk thread attached to the ceiling, and then opens a window to allow in a stream of sunlight.  "It was never known what the old Jew said, but he whispered to the stone just as if it could hear, and then said to his son, 'Thou seest that that crystal focuses the light from heaven, and thou seest that the focus is at the end of this silken thread.  Now, this precious opal will go forth in search of truth, and it will tell thee whether this marriage, if it be undertaken, will be for thy good or not.  Thou must sit with closed eyes at the other side from the crystal; the rays from the sun will fall direct upon the gem when it is at rest; then when I tell thee to open thine eyes, mark well the colour thou first seest, and, according to that, we will settle how this matter is to be.'"  The mystic then swings the gem to and fro, like a pendulum, then leaves it to itself.  "Gradually its oscillations became less and less, until at last, just as he was getting somewhat impatient, the young man heard a quick, sharp word, 'Look!' and he opened his eyes and fixed them upon the stone.  A blue blaze of fire met his eye, blue as the heavens, bright as the sun in those heavens."  Yellow would have indicated gold.  Red would have been danger.  Blue meant: "Prize above all its heavenly hue; It guides to what is just and true."  (The Quiver, 1889, p. 871.)